The available modes of delivery of pharmaceutical and therapeutic agents often are severely limited by chemical or physical barriers or both, which are imposed by the body. For example, oral delivery of many such agents would be the route of choice if not for the presence of chemical and physicochemical barriers such as extreme pH in the gut, exposure to powerful digestive enzymes, and impermeability of gastrointestinal membranes to the active ingredient. Among the numerous pharmacological agents which are known to be unsuitable for oral administration are biologically active peptides and proteins, such as insulin. These agents are rapidly destroyed in the gut by acid hydrolysis and/or by proteolytic enzymes.
A great deal of research has been devoted to developing effective oral drug delivery methods and systems for these vulnerable pharmacological agents. The proposed solutions have included:
(a) co-administration of adjuvants (such as resorcinols and non-ionic surfactants polyoxyethylene oleyl ether and n-hexadecyl polyethylene ether to increase the permeability of the intestinal walls; and
(b) co-administration of enzymatic inhibitors, such as pancreatic trypsin inhibitor, diisopropylfluorophosphate (DFF) and trasylol to avoid enzymatic degradation.
The use of such substances, in drug delivery systems, is limited however either because of:
(a) their inherent toxicity when employed at effective amounts; PA1 (b) their failure to protect the active ingredient or promote its absorption; PA1 (c) their adverse interaction with the drug. PA1 (a) poor stability; PA1 (b) inadequate shelf life; PA1 (c) limited to low MW (&lt;30,000) cargoes; PA1 (d) difficulty in manufacturing; PA1 (e) adverse interactions with cargoes.
Liposomes as drug delivery systems have also been described. They provide a layer of lipid around the encapsulated pharmacological agent. The use of liposomes containing heparin is disclosed in U.S. Pat. No. 4,239,754 and several studies have been directed to the use of liposomes containing insulin; e.g., Patel et al. (1976) FEBS Letters Vol. 62, page 60 and Hashimoto et al. (1979) Endocrinol. Japan, Vol. 26, page 337. The use of liposomes, however, is still in the development stage and there are continuing problems, including:
More recently, synthetic amino acid polymers or proteinoids, forming microspheres, have been described for encapsulating pharmaceuticals. For example, U.S. Pat. No. 4,925,673 (the '673 patent), the disclosure which is hereby incorporated by reference in its entirety, describes such microsphere constructs as well as methods for their preparation and use. The '673 patent also describes microspheres which encapsulate pharmaceutical agents for delivery into the gastrointestinal tract or into the blood.
While the proteinoid microspheres described in the '673 patent are useful for their intended purposes, the physicochemical properties of the proteinoid microspheres, such as light sensitivity, shelf life and the selectivity of their solubility in various portions of the gastrointestinal tract, could be improved. Additionally, there is a need in the art for microspheres that can encapsulate a broader range of active agents such as polar drugs.
The method employed in the '673 patent to prepare proteinoids produces a complex mixture of high molecular weight (MW) (&gt;1000 daltons) and low MW (.ltoreq.1000 daltons) peptide-like polymers which are difficult to separate. Moreover, the method produces a small amount of the low MW proteinoids which is the microsphere-forming fraction. Hence, an improved method of preparing of the proteinoids is also desired.
Accordingly, there is a need in the art for improved proteinoid carriers as well as improved methods for their preparation.